Variable focus lenses are known, as disclosed in WO 96/38744 and WO 99/47948 for example. They normally consist of a liquid-filled chamber, at least one face of which is formed by a transparent flexible membrane, for example a Mylar membrane. As liquid is added to or removed from the chamber, the flexible membrane is deformed, and its curvature accordingly changes. This change in curvature leads to a change in the optical characteristics and power of the lens. The power of the lens can thus be varied by varying the amount of liquid in the chamber.
In WO 96/38744, the amount of fluid in the lens is adjusted by inserting a syringe through a bung, and using the syringe to add or withdraw fluid. If this lens is to be used in spectacles, the spectacle frames for the lenses must allow the lens to be accessed by the syringe. The fluid connections makes the spectacles and lenses complex and bulky. Thus, it will be appreciated that use of this lens can be rather inconvenient in some circumstances. A syringe can also be difficult to control accurately, leading to a corresponding reduction in accuracy of the adjustment of the lens
To address some of these issues, WO 99/47948 proposed various arrangements including some alternatives to the use of a syringe. These included a range of methods for displacing a syringe type piston or plunger in a reservoir to pump fluid into the lens chamber, and also the use of a barrel with an external thread, and a cap with an internal thread mounted at one end of the barrel. By using a thread, turning the cap can adjust the volume of a fluid reservoir defined within the barrel and cap, and this can be used to pump fluid into and out of the fluid chamber in the lens.
However, although these alternatives enable improvements in the ease of control of the lens, the adjustment mechanism is still bulky and complex.
Reservoirs of reduced size can be used to avoid the problem of bulk, but this limits the range of adjustment that can be applied to the lens, and thus limits the use of the lens. In order to enable the lens to be adjusted to a wide range of powers and thus address a wide range of vision problems, a reservoir of large size must be used.
In an attempt to provide this capability without a cumbersome spectacle design, WO 99/47948 discloses mechanisms for sealing and disconnecting the reservoir. This means that a large reservoir can be removed from the lens after the desired adjustment has been carried out. In one embodiment a screw thread is used to compress a pipe to seal it, and the pipe is then cut to allow a reservoir to be removed. In another embodiment the device for compressing the pipe is a button that is pressed onto the pipe. The button includes a cutting blade, that presses onto and cuts the pipe once depression of the button has compressed and sealed the pipe. However, whilst these mechanisms enable removal of the large fluid reservoir, they still require a complex and inconvenient connection between the reservoir and the lens, as well as a complicated procedure to remove the reservoir.